Plasma reactor having a symmetric parallel conductor coil antenna

ABSTRACT

The invention in one embodiment is realized in a plasma reactor for processing a semiconductor workpiece. The reactor includes a vacuum chamber having a side wall and a ceiling, a workpiece support pedestal within the chamber and generally facing the ceiling, a gas inlet capable of supplying a process gas into the chamber and a solenoidal interleaved parallel conductor coil antenna overlying the ceiling and including a first plurality conductors wound about an axis of symmetry generally perpendicular to the ceiling in respective concentric helical solenoids of at least nearly uniform lateral displacements from the axis of symmetry, each helical solenoid being offset from the other helical solenoids in a direction parallel to the axis of symmetry. An RF plasma source power supply is connected across each of the plural conductors.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 09/611,169, filed Jul.6, 2000, now U.S. Pat. No. 6,685,798, entitled “A PLASMA REACTOR HAVINGA SYMMETRICAL PARALLEL CONDUCTOR COIL ANTENNA”, by John Holland, et al.and assigned to the present assignee.

The following application/patents contain subject matter related to thepresent invention:

U.S. patent application Ser. No. 09/611,170, filed Jul. 6, 2000,entitled “A PLASMA REACTOR HAVING A SYMMETRIC PARALLEL CONDUCTOR COILANTENNA”, by John Holland, et al.; U.S. Pat. No. 6,409,933, issued Jun.25, 2002, entitled “A PLASMA REACTOR HAVING A SYMMETRICAL PARALLELCONDUCTOR COIL ANTENNA”, by John Holland, et al.; U.S. Pat. No.6,414,648, issued Jun. 11, 2002, entitled “A PLASMA REACTOR HAVING ASYMMETRICAL PARALLEL CONDUCTOR COIL ANTENNA”, by John Holland, et al.;U.S. Pat. No. 6,462,481, issued Oct. 8, 2002, entitled “A PLASMA REACTORHAVING A SYMMETRIC PARALLEL CONDUCTOR COIL ANTENNA”, By John Holland, etal.

BACKGROUND OF THE INVENTION

Plasma reactors used to fabricate semiconductor microelectronic circuitscan employ RF inductively coupled fields to maintain a plasma formedfrom a processing gas. Such a plasma is useful in performing etch anddeposition processes. Typically, a high frequency RF source power signalis applied to a coil antenna near the reactor chamber ceiling. Asemiconductor wafer or workpiece support on a pedestal within thechamber has a bias RF signal applied to it. The power of the signalapplied to the coil antenna primarily determines the plasma ion densitywithin the chamber, while the power of the bias signal applied to thewafer determines the ion energy at the wafer surface. One problem withsuch a coil antenna is that there is a relatively large voltage dropacross the coil antenna, which can induce unfavorable effects in theplasma such as arcing. This effect becomes more acute as the frequencyof the source power signal applied to the coil antenna is increased,since the reactance of the coil antenna is proportional to frequency. Insome reactors, this problem is addressed by limiting the frequency to alow range such as about 2 MHz. Unfortunately, at such lower frequencies,the coupling of RF power to the plasma can be less efficient. It isoften easier to achieve a stable high density plasma discharge atfrequencies in the range of 10 MHz to 20 MHz. Another disadvantage ofoperating at the lower frequency range (e.g., 2 MHz) is that thecomponent size of such elements as the impedance match network are muchlarger and therefore more cumbersome and costly.

Another problem with coil antennas is that efficient inductive couplingto the plasma is generally achieved by increasing the number of turns inthe coil which creates a larger magnetic flux density. This increasesthe inductive reactance of the coil, and, since the circuit resistance(consisting primarily of the plasma resistance) remains constant, thecircuit Q (the ratio of the circuit reactance to resistance) increases.This in turn leads to instabilities and difficulties in maintaining animpedance match over varying chamber conditions. Instabilities ariseparticularly where the coil inductance is sufficiently great so that, incombination with stray capacitance, self-resonance occurs near thefrequency of the RF signal applied to the coil. Thus, the inductance ofthe coil must be limited in order to avoid these latter problems.

These problems have been largely solved by the invention of an inductivecoil antenna having multiple interleaved symmetrically arrangedconductors spiraling outwardly as set forth in U.S. Pat. No. 5,919,389,filed Jul. 6, 1999 entitled “INDUCTIVELY COUPLED PLASMA REACTOR WITHSYMMETRICAL PARALLEL MULTIPLE COILS HAVING A COMMON RF TERMINAL”, byXue-Yu Qian et al. By dividing the antenna into multiple conductors inan interleaved symmetric pattern, the voltage drop is reduced because itis divided among plural conductors of the antenna. Thus, the frequencyof the source power signal is not restricted as in a conventional coilantenna. This type of coil antenna is referred to in this specificationas an “interleaved” coil antenna. Such an interleaved coil antenna isdisclosed in various configurations including a flat pancake shape aswell as a dome shape or a dome shape with a cylindrical skirt around theside walls or a flat pancake shape with cylindrical skirts around thechamber side wall (U.S. Pat. No. 5,919,389).

One limitation of coil antennas overlying the chamber ceiling (bothconventional as well as the interleaved type) is that the mutualinductance between adjacent conductors in the antenna is generally in ahorizontal direction generally orthogonal from the vertical direction inwhich RF power must be inductively coupled to the plasma. This is oneimportant factor that limits the spatial control of the power depositionto the plasma. It is a goal of the present invention to overcome thislimitation in the spatial control of the inductive coupling.

Typically with “inner” and “outer” coil antennas, they physically aredistributed radially or horizontally (rather than being confined to adiscrete radius) so that their radial location is diffused accordingly.This is particularly true of the horizontal “pancake” configuration.Thus, the ability to change the radial distribution of plasma iondistribution by changing the relative apportionment of applied RF powerbetween the inner and outer antennas is limited. This problem isparticularly significant in processing semiconductor wafers with largerdiameters (e.g., 300 mm). This is because as the wafer size increases,it becomes more difficult to maintain a uniform plasma ion densityacross the entire wafer surface. The radial distribution of plasma iondensity can be readily sculpted by adjusting the radial distribution ofthe applied magnetic field from the overhead antenna. It is this fieldwhich determines plasma ion density. Therefore, as wafer size increases,a greater ability to sculpt or adjust the radial distribution of theapplied RF field is required. Accordingly, it would be desirable toenhance the effect of the apportionment of applied RF power between theinner and outer antennas, and in particular to accomplish this byconfining each of the inner and outer antennas to discrete or verynarrow radial locations.

Another problem encountered with the use of inner and outer coilantennas is that the outer antenna typically has a significantly greaterinductance than the inner antenna (because of the longer distances atthe outer radii), so that they have vastly different impedances. As aresult, the impedances of the two coils are not similar. This problem ismore acute as the chamber size increases to accommodate the trend towardlarger semiconductor wafers. One way around this problem is to useindependent RF power sources to drive the inner and outer antennas.Since each power source has its own impedance match network, a disparitybetween the impedances of the inner and outer antennas is not a problem.However, another problem arises in that it is difficult or impracticalto keep the two independent power sources in phase, so that undesirableeffects arise due to the occurrence of constructive and destructiveinterference between the RF magnetic fields generated by the twoantennas as their RF currents wander in and out of phase. This problemis overcome in accordance with one aspect of the invention by employinga novel dual output RF power source having the ability to apportiondifferent RF power levels to its two outputs. However, with such asingle RF source, the disparity between the impedances of the inner andouter antennas is again a problem. It would therefore be desirable tofacilitate at least near equalization of the impedances of the inner andouter coils without sacrificing the inductive coupling of either.

SUMMARY OF THE DISCLOSURE

One embodiment of the invention is realized in a plasma reactor forprocessing a semiconductor workpiece, the reactor including a vacuumchamber having a side wall and a ceiling, a workpiece support pedestalwithin the chamber and generally facing the ceiling, a gas inlet capableof supplying a process gas into the chamber and a solenoidal interleavedparallel conductor coil antenna overlying the ceiling and including afirst plurality conductors wound about an axis of symmetry generallyperpendicular to the ceiling in respective concentric helical solenoidsof at least nearly uniform lateral displacements from the axis ofsymmetry, each helical solenoid being offset from the other helicalsolenoids in a direction parallel to the axis of symmetry. A RF plasmasource power supply is connected across each of the plural conductors.

In another embodiment, the antenna is a solenoidal segmented parallelconductor coil antenna overlying the ceiling and including a firstplurality conductors wound about an axis of symmetry generallyperpendicular to the ceiling in respective concentric side-by-sidehelical solenoids, each helical solenoid being offset by a distance onthe order of a conductor width of the plurality of conductors from thenearest other helical solenoids in a direction perpendicular to the axisof symmetry, whereby each helical solenoid has slightly differentdiameter.

In either embodiment, the reactor may further include an inner coilantenna overlying the ceiling and surrounded by and having a lateralextent less than the first solenoidal interleaved parallel conductorcoil antenna, whereby the first parallel conductor coil antenna is anouter coil antenna. In one implementation, the reactor further includesa second RF plasma source power supply connected to the inner coilantenna whereby the respective RF power levels applied to the inner andouter antennas are differentially adjustable to control radialdistribution of the applied RF field from the inner and outer antennas.However, in a preferred implementation, the RF plasma source powersupply includes two RF outputs having differentially adjustable powerlevels, one of the two RF outputs being connected to the outer antennaand the other being connected to the inner antenna, whereby therespective RF power levels applied to the inner and outer antennas aredifferentially adjustable to control radial distribution of the appliedRF field from the inner and outer antennas.

Preferably, the number of the first plurality of parallel conductors isgreater than the number of the second plurality of parallel conductorsand the lengths of the first plurality of parallel conductors areshortened accordingly, so as to bring the inductive reactance of theouter antenna at least nearer that of the inner antenna.

If the inner antenna is also a parallel conductor antenna, thenpreferably the number of the first plurality of parallel conductors isgreater than the number of the second plurality of parallel conductorsand the lengths of the first plurality of parallel conductors areshortened accordingly, so as to bring the inductive reactance of theouter antenna at least nearer that of the inner antenna.

The lateral displacements of the first plurality of conductors of theouter antenna preferably are uniform and the lateral displacements ofthe second plurality of conductors of the inner antenna preferably areuniform, whereby the inner and outer antennas are confined withinrespective narrow annuli of widths corresponding to the thickness of theconductors, whereby to maximize the differential effect of the inner andouter antennas on the radial distribution of applied RF field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of the invention having a singlesolenoidal interleaved plural conductor coil antenna.

FIGS. 2, 3 and 4 are perspective, top and elevational views,respectively, of a second embodiment of the invention having inner andouter solenoidal interleaved plural conductor coil antennas.

FIG. 5 is a perspective view of a first preferred embodiment of theinvention having inner and outer solenoidal interleaved conductor coilantennas.

FIGS. 6A and 6B are perspective and top views, respectively, of anotherembodiment of the invention having a single solenoidal segmented pluralconductor coil antenna.

FIG. 7A illustrates a further embodiment of the invention having innerand outer solenoidal segmented conductor antennas.

FIG. 7B illustrates a modification of the embodiment of FIG. 7A in whichthe coil antennas conform with a dome shape.

FIG. 8 illustrates yet another embodiment of the invention including anouter flat interleaved conductor coil antenna whose conductor lengthsare tuned to more nearly match the impedance of the inner coil antenna.

FIGS. 9-13 illustrate various configurations of solenoidal interleavedconductor coil antennas with plasma reactors having dome-shaped reactorchamber ceilings.

FIGS. 14 and 15 illustrated various configurations of solenoidalinterleaved plural conductor coil antennas with plasma reactors havingflat reactor chamber ceilings.

FIG. 16 illustrates an embodiment of the invention combininginterleaving and segmenting of plural conductors in a single solenoidalcoil antenna.

FIG. 17 illustrates a preferred embodiment of the invention having innerand outer coil antennas, in which the outer antenna is a solenoidal coilantenna of the type illustrated in FIG. 16 having interleaved andsegmented conductors.

FIG. 18 illustrates a single power source having dual differentiallyadjustable outputs connected respectively to the inner and outer coilantennas of FIG. 5.

FIG. 19 illustrates dual output power source of FIG. 18 connected to theinner and outer coil antennas of FIG. 7.

FIG. 20 illustrates the dual output power source of FIG. 18 connectedrespectively to the inner and outer coil antennas of FIG. 8.

FIG. 21 illustrates a further embodiment of the invention having inner,intermediate and outer solenoidal plural conductor coil antennas.

FIG. 22 illustrates a first embodiment of a differentially adjustablethree-output RF power source for use with the reactor of FIG. 21.

FIG. 23 illustrates a second embodiment of a differentially adjustablethree-output RF power source for use with the reactor of FIG. 21.

FIG. 24 illustrates a version of the embodiment of FIG. 1 in which thecoil antenna is rectangular rather than circular.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Solenoidal Interleaved Coil Antenna:

Referring to FIG. 1, efficiency of inductive coupling to the plasma isenhanced by constructing the antenna 100 as a solenoidal multi-conductorinterleaved coil antenna. In the illustrated embodiment, the solenoidalantenna 100 defines a vertical right circular cylinder or imaginarycylindrical surface or locus whose axis of symmetry coincides with thatof the reactor vacuum chamber 101. It preferably further coincides withthe axis of symmetry of a workpiece which may be accepted forprocessing. In FIG. 1, the reactor chamber 101 is defined by acylindrical side wall 105 and a flat ceiling 110. A wafer supportpedestal 115 is provided within the reactor chamber 101, oriented infacing relationship to the chamber ceiling and centered on the chamberaxis of symmetry. A vacuum pump 120 cooperates with an exhaust outlet ofthe chamber. A process gas supply 125 furnishes process gas into thereactor chamber interior through a gas inlet 130. The process gas maycontain a halide gas for polysilicon etching, a fluorocarbon gas forsilicon dioxide etching, or saline gas for a silicon chemical vapordeposition process, for example. Or, the gas may contain achlorine-bearing gas for metal etching, for example. The gas inlet 130is illustrated in FIG. 1 as a single pipe but in practical applicationmay be implemented through more elaborate structures such as multipleinlets.

Under the influence of RF power induced into the chamber from theantenna, such gases will support a plasma for processing the workpiece.Plasma processes which may be performed can include not only etch, butalso deposition such as chemical vapor deposition, with the use ofsuitable precursor gases.

The pedestal 115 includes a conductive electrode 115 a coupled throughan impedance match network 140 to a bias RF power source 145. Thechamber side wall 105 may be a metal such as aluminum while the ceiling110 may be a dielectric such as quartz. In other embodiments of theinvention, the ceiling 110 is not flat but may be dome shaped orconical. Moreover, the ceiling 110 may be a semiconductor rather than adielectric, the semiconductive material of the ceiling 110 being of anoptimum conductivity which enables it to act as a window to the RFinductive field from the antenna 100 as well as an electrode. How todetermine the optimum conductivity for this purpose is disclosed in U.S.Pat. No. 6,077,384, issued Jun. 20, 2000 entitled “PARALLEL PLATEELECTRODE PLASMA REACTOR HAVING AN INDUCTIVE ANTENNA COUPLING POWERTHROUGH A PARALLEL PLATE ELECTRODE”, by Kenneth S. Collins. In thiscase, where the ceiling 100 may be employed as an electrode, it may begrounded (as indicated in dashed line) or may be connected through amatch network 150 to an RF power source 155, also indicated in dashedline. The chamber and/or antenna may have a shape other thancylindrical; for example it may be rectangular, and may have a squarecross section. Workpieces also may be other than circular; for examplethey may be of square or other outer shape. Workpieces to be processedmay be semiconductor wafers, or they may be other items such as maskreticles.

The interleaved solenoidal coil antenna 100 can include any number ofinterleaved conductors. In the embodiment of FIG. 1, the coil antennaconsists of three interleaved symmetrically arranged conductors 160,163, 166. The plural conductors of the antenna lie along respectivehelical paths generally paralleling each other. Each such helix conformswith the same imaginary right cylindrical surface, forming thesolenoidal configuration. As illustrated, the helical conductors 160,163, 166 are offset uniformly from one another in the verticaldirection. More generally, the conductors are offset substantiallyuniformly from one another generally in the direction of the chamberaxis of symmetry. Their power input taps 160 a, 163 a, 166 a,respectively, are connected through an impedance match network 170 to anRF plasma source power supply 175. Their return taps 160 b, 163 b and166 b, respectively, are connected to ground. As illustrated, the powertaps 160 a, 163 a, 166 a preferably lie in the same horizontal plane inan imaginary circle, and are located along the circumference of thatimaginary circle at uniform intervals which, in the case of threeconductors, is 120 degree. More generally, the aforesaid plane istransverse to the chamber axis of symmetry. Similarly, the return taps160 b, 163 b, 166 b are co-planar and disposed at uniform intervals (120degree). In this embodiment, the helical path of each conductor 160,163, 166 slopes sufficiently in the axial direction to realize thegenerally uniform axial displacement between conductors while permittingthe taps 160 a, 163 a and 166 a to be co-planar. In other embodiments,the taps need not be co-planar.

In the embodiment of FIG. 1, the power tap and the return tap of eachconductor are axially aligned (here, vertically aligned, since thechamber axis is shown as vertically oriented). For example, the powerand return taps 160 a, 160 b of the conductor 160 are axially aligned.Preferably, the grounded ends of the windings are nearest the chamberceiling 110, as illustrated in FIG. 1, in order to keep high potentialsaway from the plasma, and thereby minimizing any tendency for arcing andundesired capacitive coupling effects.

A principal advantage is that the inductive coupling is performed byplural conductors (e.g., the three conductors 160, 163, 166) rather thanby a single conductor, so that for the same amount of inductivecoupling, shorter conductor lengths may be employed. This featuregreatly reduces the electrical potential drop along each conductor, andadvantageously reduces capacitive coupling.

In this illustrated embodiment, the antenna 100 is symmetricallyarranged about the axis of symmetry of the cylindrical reactor chamberside wall 105. Thus, for example, the input taps 160 a, 163 a, 166 a atthe top of the antenna 100 are spaced equally from the axis of symmetryof the cylindrical side wall 105 and from each other. Similarly, theoutput taps 160 b, 163 b, 166 b at the bottom of the antenna 100 arespaced equally from the axis of symmetry of the cylindrical side wall105 and from each other. Moreover, each conductor 160, 163, 166 issubstantially the same shape, substantially evenly spaced with respectto each other about the axis of symmetry, and substantially of the samelength. Preferably, the input and output taps of each conductor (e.g.,the input and output taps 160 a, 160 b) are in vertical alignment withone another (i.e., along the axis of symmetry of the cylindrical sidewall 105).

How the Solenoidal Coil Provides Better Coupling:

The solenoidal feature of illustrated embodiments of the inventionincreases the coupling to the plasma of the antenna because eachconductor segment is displaced from its nearest neighbor conductorsegment in the direction of the axis of symmetry. In this way themagnetic lines attributable to mutual coupling between the conductorsegments are in the axial direction, so that they advantageously reachtoward the plasma in the reactor chamber. Thus, coupling to the plasmais enhanced relative to designs in which the coils are flat with mutualcoupling in the direction perpendicular to the chamber axis. In theembodiment of FIG. 1, the three conductors 160, 163, 166 are displacedaxially from one another so that the mutual inductance between nearestneighbor conductors is generally in the chamber axial direction.

Inner and Outer Solenoidal Coil Antennas with Multiple InterleavedConductors:

FIGS. 2-4 illustrate perspective, top and elevational views,respectively, of a reactor having inner and outer solenoidal antennaseach having interleaved multiple conductors of the type illustrated inFIG. 1. An inner solenoidal antenna 210 has two interleaved conductors215, 220 (rather than three as in FIG. 1). However, in otherembodiments, a greater number of such interleaved conductors may beprovided. The power terminals 215 a, 220 a are disposed at 180 degreeangular separations from each other, as are the return terminals 215 b,220 b. As in the embodiment of FIG. 1, the power and return terminals ofeach conductor 215, 220 of FIG. 2 are in vertical alignment, although inother implementations they may not be in axial alignment. Also as in theembodiment of FIG. 1, in FIG. 2 the power taps 215 a, 220 a lie in a topplane transverse to the axis while the return taps 215 b, 220 b lie in abottom plane transverse to the axis. In the illustrated position, bothof these transverse planes are horizontal. Each of the conductors 215,220 is wound in a helix having a sufficient slope so that the 180 degreeangular separation of the power taps 215 a, 220 a is sufficient toprovide the axial offset between the conductors 215, 220 illustrated inFIG. 2.

An outer antenna 230 has three interleaved parallel conductors 235, 240,245 with power taps 235 a, 240 a, 245 a at 120 intervals in the tophorizontal plane and return taps 235 b, 240 b, 245 b at 120 intervals inthe bottom horizontal plane. In order to facilitate adjustment of theradial distribution of plasma ion density, the power levels applied toeach one of the inner and outer antennas 210, 230 must be separately ordifferentially adjustable. For this purpose, FIG. 2 illustrates twoseparate RF power sources 250, 255 coupled to the inner and outerantennas 210, 230 through respective impedance match networks 260, 265.One problem using separate power sources is that their output signalsmay tend to wander in and out of phase. As an alternative, FIG. 3illustrates a common RF power source 270 with differentially adjustabledual outputs connected to the inner and outer antennas 210, 230. Thedual output RF power source 270 is described later in thisspecification. Its principal advantage is that the separately adjustableRF signals applied to the inner and outer antennas 210, 230 are inphase, but their respective power levels may be adjusted relative to oneanother. The innovative design of the multiple coil antenna facilitatesimpedance matching and balancing as between the multiple coils, and theuse of a common power source.

The elevational cut-away view of FIG. 4 shows how the discrete radialconfiguration of the inner and outer antennas 210, 230 overlies such asmall area of the ceiling 110 that the remaining area provides more thansufficient space for the placement of temperature control elements overmost of the ceiling area. Specifically, for example, the temperaturecontrol elements may include thermally conductive spacers 286, 288contacting the top surface of the ceiling 110 at portions not underlyingthe inner and outer antennas 210, 230. The inner spacer 286 is a solidright cylinder surrounded by the inner antenna 210, while the outerspacer is a solid annulus flanked by the inner and outer antennas 210,230. A cooling plate 290 overlies and contacts the top surfaces of thethermally conductive spacers 286, 288 and has coolant passages 292extending therethrough in which a liquid coolant may be circulated.Furthermore, the spacers 286, 288 may have hollow spaces to accommodateheater lamps 294 facing the ceiling 110.

How the Solenoidal Inner/Outer Antennas Increase the Adjustment of theRadial Distribution of Plasma Ion Density:

Inner and outer antennas of the flat (“pancake”) type tend to bedistributed across a relatively large horizontal annulus so that theirradial power deposition “locations” are not discretely defined. Forexample, some of the outer windings of the inner antenna are near theinner windings of the outer antenna. Thus, these RF currents flowing inthe outermost windings of the inner antenna will have an effect on thecoupling of inner windings of the outer antenna. Likewise, RF currentflowing in the innermost windings of the outer antenna will have aneffect on the coupling of the outer windings of inner antenna. As aresult, the positional effect of the inner and outer antennas isdiffused and the radial power distribution can not be easily controlledby simply adjusting the RF power applied to each coil. This reduces theextent to which they can shift the radial distribution of the RF field(and therefore of the radial distribution of the plasma ion density) fora given difference between the power levels applied to the inner andouter antennas.

In contrast, in the embodiment illustrated in FIG. 24, the solenoidalinner and outer antennas 210, 230 whose plural conductors are offsetfrom each other generally in the vertical direction (or more generallyin the direction of the chamber axis) have virtually no radial widthbeyond that of the thin conductors themselves. This is best seen in theembodiment of FIG. 3, clearly showing that in the horizontal plane (ormore generally a plane transverse to the chamber axis) the inner andouter antennas 210, 230 appear as two discrete concentric circles whosecircular lines are thin. Thus, for example, all of the RF power appliedto the outer antenna 230 radiates into the chamber from the location ofthe single discrete radius of the outer antenna, so that none of it is“wasted” at interior radial locations as in the conventional antennamentioned above. The same is true of the inner antenna 210 in that allof the RF power applied to the inner antenna 210 radiates from thesingle discrete radius of the inner antenna 210. Thus, none of it is“wasted” at exterior radial locations. As a result, for a given range ofdifferences in applied power levels on the inner and outer antennas 210,230, a much greater shift in radial distribution of plasma ion densityis realized than in the conventional case.

This feature provides a great advantage as the chamber size is scaledupwardly to accommodate larger semiconductor wafer sizes. As the wafersize increases, it becomes more difficult to maintain a uniform plasmaion density across the entire wafer surface or to adjust thedistribution of the plasma ion density across the wafer surface. Theradial distribution of plasma ion density is in large measure determinedby the radial distribution of the applied inductive field. Therefore,the radial distribution of plasma ion density can be readily sculpted byadjusting the radial distribution of the applied inductive field fromthe overhead antenna. As wafer size increases, a greater ability tosculpt or adjust the radial distribution of the applied RF inductivefield is required than previously possible. This need is now met byenhancing the effect of the apportionment of applied RF power betweenthe inner and outer antennas, by: (a) confining each of the inner andouter antennas to discrete or very narrow radial locations, and (b)providing each of such antennas as plural symmetrically arrangedconductors. This provides the basis for significantly enhanced impedancematching of different diameter antennas and power-apportioningcapability, as well as minimizing voltage drop and undesired capacitivecoupling effects, as set out in more detail below.

How the Impedances of the Inner and Outer Antennas are Matched:

As mentioned previously in this specification, the larger dimensions ofthe outer antenna 230 dictate longer conductor lengths and thereforegreater inductive reactance than the inner antenna 210. This createsproblem in maintaining uniform potential differences across the reactorchamber and creates an impedance match problem if a common RF powersource is employed. One aspect of the invention overcomes this problemby adjusting the length and number of the plural conductors in theinterleaved coils of the inner as compared to the outer antenna. Inparticular, the outer antenna is provided as a greater number ofindividual interleaved conductors than the inner antenna. Moreover, eachof the conductors of the outer antenna is proportionately shorter. Theproportion of the number of interleaved conductors and conductor lengthsbetween the inner and outer antennas is sufficient to reduce thedisparity between the impedances of the inner and outer antennas.

Thus, the problem is solved in one aspect of the invention by reducingthe inductance (length) of each of the conductors in the outer antenna230. In order to avoid a concomitant reduction in the overall inductivecoupling of the outer antenna 230, a greater number of individualconductors is provided in the outer antenna 230 than in the innerantenna 210. Specifically, while the inner antenna 210 has only twoconductors with taps disposed at 180, the outer antenna 230 has threeconductors with taps disposed at 120, as shown in FIGS. 2-4. The greaternumber of conductors for the other antenna enhances inductive couplingin order to compensate for the shorter individual conductor length.Further, each of the shorter conductors exhibits a much reduced voltagedrop as compared with the use of a similar single conductor antenna,thus cutting undesired capacitive coupling effects.

First Integrated Embodiment:

FIG. 5 illustrates a first integrated embodiment having multiplesolenoidal overhead antennas, each having a plurality of interleavedconductors. An inner solenoidal antenna 510 has a pair of interleavedconductors 515, 520 with power taps 515 a, 520 a at 180 intervals. Anouter solenoidal antenna 525 has four interleaved conductors 530, 535,540, 545 with power taps 530 a, 535 a, 540 a, 545 a at 90 degreeintervals with respect to the axis of symmetry. Each interleavedconductor is generally parallel to the remaining conductors of a givenantenna. An inner circular power bus 550 overlying the inner antenna 510is connected to the inner antenna power taps 515 a, 520 a. Similarly, anouter circular power bus 552 overlying the outer antenna 525 isconnected to the outer antenna power taps 530 a, 535 a, 540 a, 545 a. Aset of four arms 560, 562, 564, 566 underlying the outer antenna 525 anddisposed at 90 degree intervals connect respective ground taps to acircular grounded housing 570. Two of the arms 560, 564 opposing oneanother at 180 degree intervals are connected to the inner antennaground taps 515 b, 520 b, respectively and to outer antenna ground taps530 b, 540 b. The remaining two opposing arms 562, 566 are connected tothe outer antenna ground taps 535 b, 545 b. For each one of the pluralconductors of a given antenna in FIG. 5, the power tap and the groundtap are in axial alignment.

Further, the power and ground taps of both the inner and outer antennasare colinear, and in axial alignment, although alternative embodimentsare possible in which they need not be aligned. The multiple conductorsand symmetric design facilitates the use of such aligned taps bothwithin each individual coil and as between multiple coils, greatlysimplifying RF power input to the antennas and minimizing cross-talk,stray reactances, and the possibility of nonuniformities in the plasma.

Segmented Side by Side Solenoidal Conductors:

FIGS. 6A and 6B illustrate an alternative embodiment of a singlesolenoidal plural-conductor coil antenna in which the plural conductorsare not interleaved (as in the type of coil illustrated in FIG. 1 forexample), but rather are segmented into parallel side-by-side conductors610, 620, thus forming a solenoidal antenna which can be thought of ascomprised of individual side by side segmented conductors. The top viewof FIG. 6B clearly shows how such segmented conductors are side-by-side,rather than being displaced axially in the direction of the chamber axisor as illustrated, vertically. As in the interleaved embodiments, theside by side plural conductors of a given antenna are also symmetricallyarranged about the axis along helical paths substantially parallel toeach other. One of the conductors 610, 620 has a slightly larger helicalradius than the other, so that the conductor 610 is the inner segmentand the conductor 620 is the outer segment. The side-by-side conductors610, 620, function, however, as a single antenna because they areclosely spaced together. For example, in the illustrated embodiment,they are spaced apart by a radial distance within a factor of 20 timesthe thickness of the conductors 610, 620. In some implementations, thisdistance may be as large as 30 times the conductor thickness or aslittle as a fraction of the conductor thickness.

FIG. 7A illustrates how two solenoidal segmented side by side pluralconductor antennas of the type illustrated in FIGS. 6A and 6B may beused as the inner and outer antennas in lieu of the inner and outerantennas of FIG. 5. In FIG. 7A, an inner antenna 710 consists of a pairof side-by-side solenoidal conductors 712, 714 with power taps 712 a,714 a at the top and return taps 712 b, 714 b at the bottom. An outerantenna 730 consists of four side-by-side solenoidal conductors 735,740, 745, 750, each having a smaller number of conductors than those ofthe inner antenna 710. Their power taps 735 a, 740 a, 745 a, 750 a areat the top and their return taps 735 b, 740 b, 745 b, 750 b are at thebottom. The power taps of the inner and outer antennas 710, 730 arepreferably connected to different power output terminals so that theirpower levels may be adjusted differentially. This may be accomplishedusing separate power supplies or a common power supply with separatelyor differentially adjustable outputs, as will be described below.

FIG. 7B illustrates a version of the embodiment of FIG. 7A in which thereactor chamber ceiling 110, rather than being flat as in the embodimentof FIG. 7A, is dome-shaped, and the segmented solenoidal inner and outercoil antennas 710, 730 conform to the dome-shaped ceiling 110 of FIG.7B. Thus, each solenoidal coil 712, 714 of the inner antenna 710 andeach solenoidal coil 735, 740 of the outer antenna 730 are wound in aconical helix or helical dome shape, in which the lower windings of eachcoil 712, 714, 735, 740 have a greater diameter than the higher windingsof the coil. Preferably, the conical surface followed by the coils 712,714, 735, 740 are congruent with the dome-shaped ceiling 110 of FIG. 7B.

Tuning Inner and Outer Flat Coil Antennas:

FIG. 8 illustrates how a flat version of the inner and outer interleavedcoil antennas may be modified to tune them so as to bring theirimpedances nearer a match. As in the embodiment of FIG. 5, the innerantenna 810 of FIG. 8 has two interleaved conductors 815, 820, while theouter antenna 825 has four interleaved conductors 830, 835, 840, 845.The power taps 815 a, 820 a of the inner antenna are commonly connectedwhile the ground taps 815 b, 820 b are disposed at 180 degree intervals.The power taps 830 a, 835 a, 840 a, 845 a of the outer antenna aredisposed at 90 degree intervals, as are the outer antenna ground taps830 b, 835 b, 840 b, 845 b. As in the embodiment of FIG. 5, the innerand outer antennas of FIG. 8 are nearly matched in impedance because theouter antenna has been provided as twice as many individual conductorsas the inner antenna, whose lengths are therefore shortenedproportionately to reduce their individual inductances withoutsacrificing the overall inductive coupling of the outer antenna.

As referred to above, a better impedance match between the inner andouter multiple conductor antennas 810, 825 facilitates numerousdesirable advantages, including superior coupling of power into theplasma and a more practical adaptation to use with a common power sourcefor both antennas. The same principles of improved impedance matchshould apply to inductive sources having plural antennas, eachcomprising multiple conductors, regardless of configuration, includingboth solenoidal and flat, as well as interleaved and segmented.

Solenoidal Interleaved Antennas with Dome Ceilings:

FIG. 9 illustrates how a plasma reactor in which the ceiling 110 isdome-shaped can have the cylindrical solenoidal inner and outer antennas510, 525 of FIG. 5. In FIG. 9, the outer antenna 525 rests on an outersection of the dome ceiling and therefore is at a somewhat lower levelthan the inner antenna 510.

FIG. 10 illustrates a version of FIG. 9 in which the outer antenna 525is modified to be a conformal antenna 525′ that conforms with thesloping and nearly vertical surface of the outer section of thedome-shaped ceiling 110.

FIG. 11 illustrates a version of FIG. 9 in which the solenoid of theouter winding 525 is modified to be an antenna 525″ having an invertedconical sectional shape to it so that the cross-section is perpendicularto the surface of the dome-shaped ceiling 110.

FIG. 12 illustrates a version of FIG. 10 in which the inner antenna 510is replaced by a flat interleaved coil antenna 1200 of the typedisclosed in the above-referenced patent to Qian et al.

FIG. 13 illustrates a version of FIG. 9 in which the outer antenna 525is placed at the level of the cylindrical side wall 105 so that itsurrounds the side wall 105 rather than overlying the ceiling 110.

Solenoidal Interleaved Antennas with Flat Ceilings:

FIG. 14 illustrates a version of FIG. 13 in which the ceiling 110 isflat.

FIG. 15 illustrates a version of FIG. 14 in which the inner antenna is aflat interleaved parallel conductor coil antenna 1200 of FIG. 12.

Combining Interleaving with Segmenting:

FIG. 16 illustrates a single solenoidal coil antenna 1600 having boththe interleaving described above with reference to FIG. 1 and segmentingdescribed above with reference to FIG. 6A. The antenna 1600 of FIG. 16consists of an inner segment 1605 having two interleaved parallelconductors 1610, 1620. The inner segment 1605 is essentially atwo-conductor version of the interleaved solenoidal coil of FIG. 1. Theantenna of FIG. 16 further consists of an outer segment 1630 surroundingthe inner segment 1605. The outer segment also has two interleavedparallel conductors 1640, 1650. The outer segment 1630 is also atwo-conductor version of the interleaved solenoidal coil of FIG. 1. Thetop ends of each of the conductors in FIG. 16 are power taps, all ofwhich are connected through an impedance match network 1660 to an RFpower source 1670. The bottom ends of each of the conductors in FIG. 16are return taps which are connected to ground.

FIG. 17 illustrates a second illustrated embodiment of the inventionsimilar to the embodiment of FIG. 5 except that the outer antenna 525 isreplaced by the antenna 1600 of FIG. 16. The inner antenna 510 of FIG.17 is the same as that described above with reference to FIG. 5.

FIG. 17 provides a perspective view that affords a more detailed view ofthe antenna 1600 than the elevational view of FIG. 16. FIG. 17 showsthat the power and ground taps 1610 a, 1610 b of the inner segment'sconductor 1610 are vertically aligned and are offset by 180 from thevertically aligned power and ground taps 1620 a, 1620 b of the innersegment's other inner antenna conductor 1620. Likewise, the power andground taps 1640 a, 1640 b of the outer segment's conductor 1640 arevertically aligned and are offset by 180 from the vertically alignedpower and ground taps 1650 a, 1650 b of the outer segment's otherconductor 1650. Moreover, the taps of the inner segment 1605 are locatedat 90 relative to the taps of the outer segment 1630.

An inner annular power bus 1750 overlying the inner antenna 510furnishes RF power to each of the power taps of the inner antenna 510.An outer annular power bus 1760 overlying both the inner and outersegments 1605, 1630 of the outer antenna furnishes RF power to each ofthe power taps of the segment 1605, 1630. Insulators 1780 support all ofthe windings as shown in FIG. 17.

An RF Power Source with Plural Differentially Adjustable Outputs:

A power source having at least two differentially adjustable poweroutputs has been referred to previously in this specification, and isdisclosed in co-pending application Ser. No. 09/544,377, filed Apr. 6,2000 entitled “Inductively Coupled Plasma Source With Controllable PowerDeposition” by Barnes et al., the disclosure of which is herebyincorporated herein by reference in its entirety. FIG. 18 illustratesone embodiment of such a power source having dual outputs. In FIG. 18,an RF power source 1800 includes an RF generator 1810 connected throughan impedance match network 1815 to a series capacitor 1820 and avariable shunt capacitor 1825. A first RF output terminal 1830 of thesource 1800 is connected between the match network 1815 and the seriescapacitor 1820, while a second RF output terminal 1840 is connected tothe opposite side of the series capacitor 1820. Adjusting the variableshunt capacitor 1825 apportions more power to one output terminal or theother, depending upon the adjustment. Thus, the power levels at the twooutput terminals is differentially adjustable. As illustrated in FIG.18, the first output terminal 1830 is connected to the inner antenna 510while the other output terminal 1840 is connected to the outer antenna525 of FIG. 5. In FIG. 19, the terminals 1830, 1840 are connected to theinner and outer segmented parallel conductor antennas 710, 730,respectively, of FIG. 7. In FIG. 20, the output terminals 1830, 1840 areconnected to the flat inner and outer interleaved coil antennas 810,825, respectively, of FIG. 8. More generally, the dual output powersource of FIG. 18 may be used with any plasma reactor having inner andouter antennas, with the terminal 1830 connected to the inner antennaand the terminal 1840 connected to the outer antenna. This is true ofeach of the reactors having inner and outer antennas described abovewith reference to FIGS. 9 through 15.

The power source may have more than two differentially adjustableoutputs for use with reactors having more than two antennas. Forexample, FIG. 21 illustrates a plasma reactor having three antennas,namely an inner antenna 2110, an intermediate antenna 2120 and outerantenna 2130. Each of these three antennas may be of any type ofsuitable coil antenna, such as a flat or solenoidal single conductorcoil antenna, a flat or solenoidal interleaved parallel conductorantenna, solenoidal segmented parallel conductor antenna or acombination of different ones of the foregoing types. However, in theembodiment illustrated in FIG. 21, the inner antenna 2110 is thesolenoidal interleaved parallel conductor antenna 210 of FIG. 2, and theintermediate antenna 2120 is the segmented and interleaved parallelconductor antenna 1600 of FIG. 16. Moreover, the outer antenna 2130 is alarger version of the segmented and interleaved parallel conductorantenna 1600 of FIG. 16.

FIG. 22 illustrates an RF power source with three differentiallyadjustable output terminals for use with a three-antenna plasma reactorsuch as the three-antenna plasma reactor of FIG. 21. The RF power sourceof FIG. 22 includes an RF power generator 2210 with a match network2215, first and second series capacitors 2220, 2230 and first and secondvariable shunt capacitors 2240, 2250, the first variable shunt capacitor2240 being connected across the first series capacitor and ground andthe second shunt capacitor 2250 being connected across the second seriescapacitor 2230 and ground. A first output terminal 2260 is connectedbetween the match network 2215 and the first series capacitor 2220. Asecond output terminal 2265 is connected between the first shuntcapacitor 2240 and the second series capacitor 2230. A third outputterminal 2270 is connected to the other side of the second seriescapacitor 2230. Preferably, the first output terminal 2260 is connectedto the power taps of the inner antenna 2110 of FIG. 21, the secondoutput terminal 2265 is connected to the power taps of the intermediateantenna 2110 while the third output terminal 2270 is connected to thepower taps of the outer antenna 2130.

FIG. 23 illustrates a modified version of the three-terminal RF powersource of FIG. 22, in which the first series and shunt capacitors 2220,2240 are connected in parallel with the second series and shuntcapacitors 2230, 2250.

In practice, the variable shunt capacitors 2240, 2250 are adjusted toapportion different RF power levels to the inner, intermediate and outerantennas until the desired radial distribution of the applied RF fieldor of the plasma ion density is achieved. The particular radialdistribution to be achieved depends upon the process being performed.For example, certain processes require a uniform distribution. Otherprocesses, such as aluminum etch, produce non-uniform gas or iondistributions across the wafer surface, which can be compensated for byselecting an appropriate non-uniform radial distribution of the appliedRF field. This selection is carried out by adjustment of the variableshunt capacitors 2230, 2250.

FIG. 24 illustrates a version of the embodiment of FIG. 1 in which thecoil antenna 100 including the coiled conductors 160, 163, 166 arerectangular about the axis of symmetry rather than being circular as inthe embodiment of FIG. 1. This embodiment may be better adapted toprocessing flat panel displays or the like.

Advantages of the Disclosed Embodiments:

A number of problems in the art that have plagued plasma reactorperformance have now been overcome. The solenoidal feature of theinvention increases the efficiency of the antenna because each conductorsegment is displaced from its nearest neighbor conductor segmentgenerally in the axial direction. In this way the magnetic linesattributable to mutual coupling between the conductor segments are inthe vertical direction, so that they advantageously reach toward theplasma in the reactor chamber. Thus, coupling to the plasma is enhancedrelative to designs in which the coils are flat with mutual coupling inthe direction perpendicular to the chamber axis.

Vertical solenoidal interleaved plural conductor inner and outerantennas have virtually no radial width beyond that of the thinconductors themselves. Thus, for example, a majority of the RF powerapplied to the outer antenna radiates into the chamber from the singlediscrete radius of the outer antenna, so that none of it is “wasted” atinterior radial locations as in the conventional antenna mentionedabove. The same is true of the inner antenna in that a majority of theRF power applied to the inner antenna radiates from the single discreteradius of the inner antenna. Thus, none of it is “wasted” at exteriorradial locations. As a result, for a given range of differences inapplied power levels on the inner and outer antennas, a much greatershift in radial distribution of plasma ion density is realized than ispossible in the conventional case.

This aspect of the invention is particularly advantageous in providinguniform and/or adjustable plasma ion distribution across a very largewafer surface. Thus, the chamber size is readily scalable up to largediameter wafers using the inner/outer antenna structure. Moreover, evengreater scalability is attained by employing an even greater number ofantennas, e.g., an intermediate antenna between the inner and outerantennas.

The problem of the disparity between impedances of the inner and outerantennas is overcome by adjusting the length and number of the pluralconductors in the interleaved coils of the inner and outer antennas. Theouter antenna is divided into a greater number of interleaved conductorsthan the inner antenna. Moreover, each of the conductors of the outerantenna is proportionately shorter. The proportion of the number ofinterleaved conductors and conductor lengths between the inner and outerantennas is sufficient to reduce the disparity between the impedances ofthe inner and outer antennas. Thus, the problem is solved by reducingthe inductance (length) of each individual conductor in the outerantenna relative to the inner antenna. In order to avoid a concomitantreduction in the overall inductive coupling of the outer antenna, agreater number of individual conductors is provided in the outer antennathan in the inner antenna. The greater number of individual conductorsenhances inductive coupling in order to compensate for the shortenedconductor length in the outer antenna.

With the inner and outer antenna impedances matched or nearly matched, acommon power source to drive both antennas can be used withoutencountering impedance match problems. A illustrated embodiment of theinvention employs a common power source having multiple outputs withdifferentially adjustable power levels to permit the sculpting of theradial distribution of plasma ion density.

As an alternative to the interleaved plural conductor antenna, thesegmented plural conductor antenna enjoys the advantages of theinterleaved conductor antenna and can be implemented in the variousconfiguration disclosed above including solenoidal or dome shaped.Moreover, the segmented feature can be combined with the interleavedfeature in accordance with certain illustrated embodiments disclosedabove.

The solenoidal interleaved and segmented conductor antennas disclosedabove preferably include co-planar power taps in one (e.g., an upper)plane and co-planar return taps in another (e.g., a lower) plane. Foreach one of the plural conductors of a given antenna, its power tap andits return tap advantageously are vertically aligned (or more generally,aligned along the axis of the coil antenna), thus advantageouslysimplifying the configuration of the antenna.

Thus, for the first time, several and indeed all of the foregoingadvantages can be provided simultaneously in the same plasma source.

While the invention has been described in detail by specific referenceto illustrated embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

1. A plasma reactor for use with a supply of RF source power forprocessing a workpiece, said reactor comprising: a vacuum chamber havinga ceiling; a workpiece support pedestal within the chamber facing saidceiling and comprising a top pedestal surface having a diameter similarto a diameter of a workpiece to be supported thereon, said chamberhaving an axis of symmetry intersecting said ceiling and intersectingsaid top pedestal surface, said ceiling having a diameter greater thansaid diameter of said top pedestal surface; a first single solenoidalinterleaved coil antenna at least generally coaxial with said axis ofsymmetry, the entirety thereof overlying an intermediate portion of theceiling between a periphery of the ceiling and a center of the ceiling,the entirety of said first single solenoidal interleaved coil antennahaving a diameter substantially less than the diameter of said toppedestal surface, and comprising a first plurality of conductors woundabout said axis of symmetry in respective concentric helical solenoids,said conductors being displaced from said axis of symmetry in a lateraldirection uniformly, the conductors being offset from one another in thedirection generally of the axis of symmetry, each of said conductorsbeing connected across said supply RF source power; and an outer coilantenna overlying the ceiling and having a lateral extent greater thansaid first solenoidal interleaved conductor coil antenna, whereby saidfirst solenoidal interleaved conductor coil antenna is an inner coilantenna.
 2. The reactor of claim 1 further comprising a second RF plasmasource power supply connected to said outer coil antenna whereby therespective RF power levels applied to said inner and outer antennas aredifferentially adjustable to control radial distribution of the appliedRF field from said inner and outer antennas.
 3. The reactor of claim 1wherein said first RF plasma source power supply comprises two RFoutputs having differentially adjustable power levels, one of said twoRF outputs being connected to said outer antenna and the other beingconnected to said inner antenna, whereby the respective RF power levelsapplied to said inner and outer antennas are differentially adjustableto control radial distribution of the applied RF field from said innerand outer antennas.
 4. The reactor of claim 1 wherein said outer antennacomprises a second solenoidal interleaved conductor coil antennaoverlying the ceiling and comprising a second plurality of conductorswound about said axis of symmetry in concentric helical solenoids, andwherein the number of said second plurality of conductors is greaterthan the number of said first plurality of conductors and the lengths ofsaid second plurality of conductors are shortened accordingly, so as tobring the inductive reactance of said outer antenna at least nearer thatof said inner antenna.
 5. The reactor of claim 1 wherein said outerantenna comprises a second solenoidal interleaved conductor coil antennaoverlying the ceiling and comprising a second plurality of conductorswound about said axis of symmetry in concentric helical solenoids of atleast nearly uniform lateral displacements from said axis of symmetrybut greater than that of said inner antenna, the conductors in eachhelical solenoid being offset from the conductors in the other helicalsolenoids in a direction parallel to said axis of symmetry.
 6. Thereactor of claim 5 wherein the number of said second plurality ofconductors of said outer antenna is greater than the number of saidfirst plurality of conductors of said inner antenna.
 7. The reactor ofclaim 5 wherein the number of said second plurality of parallelconductors is greater than the number of said first plurality ofparallel conductors and the lengths of said second plurality of parallelconductors are shortened accordingly, so as to bring the inductivereactance of said outer antenna at least nearer that of said innerantenna.
 8. The reactor of claim 7 wherein the number of said secondplurality of conductors is sufficient to compensate for their shortenedlengths relative to said first plurality of conductors.
 9. The reactorof claim 8 wherein the number of said second plurality of conductors istwice the number of said first plurality of conductors.
 10. The reactorof claim 5 wherein the lateral displacements of said second plurality ofconductors of said outer antenna are uniform and the lateraldisplacements of said first plurality of conductors of said innerantenna are uniform, whereby said inner and outer antennas are confinedwithin respective narrow annuli of widths corresponding to the thicknessof said conductors, whereby to maximize the differential effect of saidinner and outer antennas on the radial distribution of applied RF field.11. The reactor of claim 10 wherein said chamber and said inner andouter antennas are cylindrical.
 12. The reactor of claim 11 wherein saidlateral displacements of said second and first pluralities of conductorsare outer and inner radii, respectively, overlying peripheral and centerregions of said chamber, respectively.
 13. The reactor of claim 5wherein: said inner coil antenna lies between top and bottom innerplanes generally perpendicular to said axis of symmetry, the helicalsolenoid defined by each conductor of said inner antenna beingterminated at a top point of the conductor near said top inner plane anda bottom point of the conductor near said bottom inner plane; said outercoil antenna lies between top and bottom outer planes generallyperpendicular to said axis of symmetry, the helical solenoid defined byeach conductor of said outer antenna being terminated at a top point ofthe conductor near said top outer plane and a bottom point of theconductor near said bottom outer plane.
 14. The reactor of claim 13wherein: said top points of said outer antenna are angularly displacedfrom one another by about 360/n, wherein n is the number of said pluralconductors of the outer coil antenna; said top points of said innerantenna are angularly displaced from one another by about 360/m, whereinm is the number of said plural conductors of the inner coil antenna. 15.The reactor of claim 14 wherein: said bottom points of said outerantenna are angularly displaced from one another by about 360/n, whereinn is the number of said plural conductors of the outer coil antenna;said bottom points of said inner antenna are angularly displaced fromone another by about 360/m, wherein m is the number of said pluralconductors of the inner coil antenna; and the top and bottom points ofeach of said conductors are in alignment along a direction parallel toaxis of symmetry.
 16. The reactor of claim 15 further comprising: aninner annular RF power conductor bus in said top inner plane and havinga radius generally the same as that of said inner antenna, said toppoints of said inner antenna being connected to said inner annular RFpower conductor bus; an outer annular RF power conductor bus in said topouter plane and having a radius generally the same as that of said outerantenna, said top points of said outer antenna being connected to saidouter annular RF power conductor bus.
 17. The reactor of claim 15wherein said top points and bottom points are spaced equally withrespect to an axis of symmetry of said reactor and with respect to oneanother.
 18. The reactor of claim 17 wherein said conductors are evenlyspaced with respect to one another and with respect to the axis ofsymmetry and are of substantially the same shape.
 19. The reactor ofclaim 14 wherein n is an integral multiple of m and wherein n/m of thetop points of said outer antenna are in angular alignment with the toppoints of said inner antenna.
 20. The reactor of claim 14 wherein theconductors of said antenna are generally mutually parallel.
 21. Thereactor of claim 1 wherein said inner coil antenna lies between top andbottom planes generally perpendicular to said axis of symmetry, thehelical solenoids defined by respective conductors being terminated atrespective top points of the conductor near said top plane andrespective bottom points of the conductor near said bottom plane, saidRF power source being connected across said top and bottom points ofeach of said conductors, wherein said top points are azimuthally equallyspaced and said bottom points are azimuthally equally spaced.
 22. Thereactor of claim 1 wherein said inner coil antenna lies between a topand bottom planes generally perpendicular to said axis of symmetry, thehelical solenoids defined by respective conductors being terminated atrespective top points of the conductors near said top plane andrespective bottom points of the conductors near said bottom plane, saidpower source being connected across said top and bottom points of eachof said conductors, wherein corresponding ones of said top and bottompoints are in axial alignment.